1. Field of the Invention
This invention relates to a sorptive separation process using crystalline zeolites. More specifically, the invention pertains to the selective separation of certain bicyclic aromatic organic compounds in admixture with other organic compounds over a particular crystalline zeolite which selectively sorbs at least one of the compounds from the mixture. The zeolites found to be useful in the present process are those having a silica/alumina mole ratio of greater than about 12, a Constraint Index of between about greater than 2 and about 12, and in particular zeolites ZSM-5, ZSM-11, ZSM-23 and ZSM-35.
2. Description of the Prior Art
It has long been known that certain porous substances such as silica gel, activated char, and zeolites, have certain selective adsorption characteristics useful in resolving a hydrocarbon mixture into its component parts. Thus, silica gel is selective in removing aromatic hydrocarbons from non-aromatic hydrocarbons and activated chars are useful in separating olefins from mixtures with paraffins. Similarly, it is well known in the art that certain crystalline zeolites can be used to separate certain hydrocarbons from feed mixtures.
U.S. Pat. No. 3,724,170 discloses chromatographic separation of C.sub.8 aromatic mixtures over zeolite ZSM-5. U.S. Pat. No. 3,699,182 to Cattanach discloses use of zeolite ZSM-5 in a process for selective separation of biphenyls from mixtures containing same and para-disubstituted aromatic isomers from mixtures containing same. Cattanach teaches a "keyhole selective sorption process wherein para-disubstituted aromatics may be selected from ortho-and/or meta-disubstituted aromatics," and also teaches that polycyclic aromatics are not sorbed by ZSM-5 type zeolites. British Pat. No. 1,420,796 shows use of zeolite ZSM-5 for adsorptive separation of p-xylene and ethylbenzene from a mixture comprised of the xylene isomers and ethylbenzene.
The ZSM-5 class of crystalline zeolites has been shown to be catalytically selective. This shape selectively can be further enhanced by the use of very large crystals, impregnation with Mg and P to reduce zeolite pore openings and coke selectivation. These modified zeolite catalysts have been very effective in such reactions as selective toluene disproportionation which yields predominantly paraxylene as the product and toluene-ethylene alkylation yielding primarily para-ethyltoluene.
Zeolite ZSM-5 possesses pore openings intermediate in size between the small pore and the large pore zeolites. It sorbs at room temperature straight chain monomethyl-substituted paraffins and monocyclic hydrocarbons at significantly faster rates than those containing dimethyl-substituted or quaternary carbon atoms, and it excludes molecules with critical dimensions larger than that of 1,3,5-trimethylbenzene. Zeolite ZSM-5 has a pore system which differentiates catalytically molecules having a straight chain, a methyl substitution and a dimethyl substitution. The catalytic properties of ZSM-5 are further elucidated by Chen and Garwood in Some Catalytic Properties of ZSM-5, a New Shape Selective Zeolite, JOURNAL OF CATALYSIS, Vol. 52, No. 3 (May 1978).
Satterfield and Cheng, Liquid Sorption Equilibrium of Selected Binary Hydrocarbon Systems in Type Y Zeolites, AICHE JOURNAL, Vol. 18, No. 4, p. 720, July 1972 and Satterfield and Smeets, Liquid Sorption Equilibria of Selected Binary Paraffin Systems in NaY Zeolite, AICHE JOURNAL, Vol. 20, No. 3, p. 618, May 1974, teach that on zeolite Y aromatic compounds are selectively adsorbed over paraffins and smaller compounds are adsorbed in preference to larger compounds. Contrary to said teaching, the zeolites for use in the instant invention yield the unexpected results of selective adsorption of paraffins over aromatics and selective adsorption of higher molecular weight molecules over smaller members of the same family.